The Descent of Man was published 11 years after On the Origin of Species, which was the culmination of decades of work, including Darwin's global voyage aboard the HMS Beagle.
But his first book, which shook the foundations of the scientific and religious world when it was published, skirted around the issue of what his theories meant for humans.
Descent of Man changed this and tackled the thorny issue head on, putting Darwin on a collision course with religion and conventional scientific thought at the time.
Fuentes points out that many of Darwin's conclusions were enlightened and stand up to modern scientific vigour, but he also condemns many of Darwin's views.
"Like so many of the scientific tomes of Darwin's day, [Descent of Man] offers a racist and sexist view of humanity," Fuentes writes. "Some of Darwin's ... assertions were dismally, and dangerously, wrong. Descent is a text from which to learn, but not to venerate."
The central theme of Darwin's work is that humans were no more special than any other animals and that we evolved from ancestral primates. But Fuentes points out Darwin applied this logic sporadically and "baselessly asserted evolutionary differences between races".
"Darwin portrayed indigenous peoples of the Americas and Australia as less than Europeans in capacity and behaviour," Fuentes says.
"Peoples of the African continent were consistently referred to as cognitively depauperate, less capable, and of a lower rank than other races."